1. Field of the Invention
The present invention relates generally to medical methods and devices, and more particularly to a method and apparatus for forming vascular prostheses from host tissue sources.
Coronary and peripheral atherosclerosis are characterized by partial or total occlusion of the arteries resulting from the accumulation of lipids, smooth muscle cells, connective tissue, and glycosaminoglycans on the arterial wall. Atherosclerosis of the coronary arteries is a particular problem and can cause angina and myocardial infarction (heart attack). Although many coronary lesions can be treated with percutaneous techniques, such as angioplasty and atherectomy, more tortuous and severely diseased arteries frequently require surgical intervention and bypass, commonly referred to as coronary artery bypass graft (CABG) surgery.
CABG surgery relies on the surgical attachment of a vascular graft to bypass the arterial occlusion in order to restore blood flow to the coronary vasculature. The nature of the vascular graft can have a significant impact on the ultimate success of the procedure. A preferred vascular graft is formed from autologous internal mammary artery (IMA), where the resulting grafts have a patency rate approaching 95% ten years following the procedure. The use of IMA grafts, however, is limited by their length, and the need to harvest the artery from the patient can result in post-surgical complications. The autologous saphenous vein is a second common source for vascular grafts. While generally available in the necessary lengths, the saphenous vein is not ideally suited for replacement as an arterial vessel, and patency rates at ten years are often below 50%. Moreover, removal of the saphenous vein from the leg can also cause post-surgical complications.
Because of the limitations on autologous vascular sources, a variety of synthetic and non-autologous biological prostheses have been proposed. Common synthetic prostheses are formed from Dacron(copyright) and PTFE, and can perform well when employed in larger diameters, i.e., above 6 mm. Smaller synthetic prostheses, however, occlude at a relatively high rate. Non-autologous biological conduits which have been utilized as vascular prostheses include human umbilical vein grafts and bovine internal mammary arteries. Synthetic grafts have also been seeded with human and other mammalian cells or proteins, e.g., collagens, in an effort to improve their long-term patency rate. Presently, however, none of these approaches has demonstrated long-term patency, particularly in smaller diameter grafts.
Of particular interest to the present invention, preparation of vascular prostheses from autologous pericardium has been proposed. Pericardial tissue is harvested from the patient and formed into a tubular graft by suturing along a longitudinal line. While promising, the use of sutures can result in an irregular seam which, in turn, can cause turbulent blood flow and result in clot formation. Moreover, such grafts are unsupported and subject to kinking and collapse. The grafts further lack an inherently round geometry and are subject to dimensional changes, e.g., elongation and aneurysmal formation. Because of the dimensional uncertainty, it is difficult to match such grafts to the precise dimensional requirements of the particular application, e.g, caliber and length. The suturing of vascular prostheses from pericardium is labor intensive and time consuming, and the resulting structures are subject to rupture and other structural failure. Thus, the outcome of using sutured pericardial tissue grafts is uncertain at best.
For these reasons, it would be desirable to provide improved vascular prostheses for use in CABG and other procedures. Such prostheses should be biocompatible with the patient, resistant to kinking and collapse, and easy to implant. Moreover, the prostheses should be non-thrombogenic, resistant to infection, and easy to sterilize and store. It would be particularly desirable to provide improved methods and apparatus for preparing vascular prostheses from autologous tissue sources, where the prostheses can be prepared in a range of diameters and lengths, and can be readily assembled in the operating room after the tissue has been harvested. In particular, the vascular prostheses should be readily assemblable, preferably without suturing, in a manner that allows precise and uniform dimensions and preferably be available in a kit form to facilitate assembly.
2. Description of the Background Art
U.S. Pat. No. 4,502,159, describes a vascular prosthesis made by suturing glutaraldehyde-treated pericardial tissue along a longitudinal seam. SU 1217362 (Abstract) describes reinforcing arteries by securing pericardial tissue over the artery. U.S. Pat. No. 3,562,820, describes forming tissue-containing prostheses over removable mandrels. The use of glutaraldehyde and other agents for treating tissue and prosthetic devices to reduce antigenicity is described in U.S. Pat. Nos. 3,988,782; 4,801,299; 5,215,541, and Brazilian applications 89/03621 and 90/03762. U.S. Pat. No. 4,539,716, describes the fabrication of an artificial blood vessel from collagen and other natural materials. U.S. Pat. Nos. 3,894,530 and 3,974,526, describe the formation of vascular prostheses from the arteries or veins present in the umbilical cord. U.S. Pat. No. 5,372,821, describes the use of tissue for forming artificial ligament grafts for use in orthopedic procedures. U.S. Pat. No. 3,408,659, describes the preparation of vascular artificial prostheses from other body lumens. French application FR 2,714,816, (Abstract) discloses a helically supported vascular prosthesis. A number of medical literature publications describe the use of vascular prostheses formed form tissue. See, for example, Rendina et al. (1995) J. Thorac. Cardiovasc. Surg. 110:867-868; Hvass et al. (1987) La Presse Mxc3xa9dicale 16:441-443; Allen and Cole (1977) J. Ped. Surg. 12:287-294; and Sako (1951) Surgery 30:148-160. Other patents and published applications relating to synthetic vascular grafts include U.S. Pat. Nos. 4,728,328; 4,731,073; 4,798,606; 4,820,298; 4,822,361; and 4,842,575; and PCT publications WO 94/22505 and WO 95/25547. Patents and published applications relating to kits for preparing replacement heart valves from pericardial and other autologous tissue sources are described in U.S. Pat. Nos. 5,163,955; 5,297,564; 5,326,370; 5,326,371; 5,423,887; and 5,425,741.
The present invention provides improved vascular prostheses and methods for their preparation. The vascular prostheses are formed in part from animal tissue, usually autologous tissue from the patient receiving the prostheses, which is supported on a separate support frame. Typically, the tissue is pericardial, fascial, rectus sheath, venous tissue, or other tissue harvested from the patient immediately before the CABG or other implantation procedure. After harvesting, the tissue is usually but not necessarily treated in a stabilizing medium, such as a cross-linking agent, and then attached to the frame in the operating room. The frame precisely defines the length and dimensions of the vascular graft and inhibits kinking and collapse of the graft after implantation. Preferably, the tissue will be tolled or otherwise formed over the frame so that adjacent longitudinal edges are overlapped to seal the resulting lumen of the graft and prevent blood leakage. In this way, suturing of the graft can be avoided.
Such vascular prostheses have a number of advantages. When using autologous tissue, the grafts are biocompatible and non-immunogenic. The grafts are durable, and use of the separate frame provides dimensional stability and inhibits unintended dilation, rupture, elongation, and kinking. Moreover, the vascular prosthesis may be prepared in a range of diameters and lengths, with the tissue sources providing a relatively unlimited source of prosthetic material. The vascular prostheses are relatively easy to fabricate, with attachment of the tissue to the frame being readily performable in an operating room environment. The frame components of the graft are easy to store and sterilize prior to use. Other advantages include non-thrombogenicity and a compliance which approximates that of natural blood vessels.
According to the method of the present invention, a tubular vascular prosthesis is formed by providing a sheet of tissue and a tubular support frame. The tissue is then attached to the tubular support frame to define a substantially unrestricted blood flow lumen therethrough. The tissue sheet may be obtained from the host or from other human or animal (non-autologous) sources. Typically, the tissue is trimmed into a shape to facilitate rolling onto the frame, usually a rectangular shape. The tissue will usually be pericardium, fascia, rectus sheath, venous tissue, or the like, and will preferably but not necessarily be treated with a cross-linking agent or other stabilizing agent (preservative) prior to formation.
The tubular support frame may have a variety of configurations. In a first embodiment, the tubular support frame includes at least an inner frame component and an outer frame component, where the attaching step comprises capturing the tissue sheet between the inner component and the outer frame component. The inner and outer frame components may be in the form of helices, longitudinally spaced-apart rings, or other conventional intravascular stent structures and the like. In a preferred aspect of the present invention, the inner and outer frame components comprise concentric mating structure which clamp the tissue therebetween without suturing. The frame thus supports the clamped tissue along the entire length of the graft to provide support and precise dimensional control.
Alternatively, the tubular support frame may include a single frame member having a plurality of fasteners disposed thereover. In such case, the attaching step comprises attaching the tissue to the fasteners, for example by penetrating the fasteners through the tissue. As yet another alternative, the tissue may be attached to a single frame using separate fasteners, such as staples which are penetrated through the tissue and into the frame. In yet another alternative, the attaching step may comprise disposing a sleeve over the tissue which in turn is disposed over the tubular frame.
Systems for forming tubular prostheses according to the present invention comprise a cutter and a tubular frame. The cutter is designed to trim the sheet of harvested tissue into a predetermined pattern, typically a rectangular pattern. The tubular frame is capable of supporting the tissue trimmed by the cutter and a tubular geometry having a substantially unrestricted flow lumen therethrough. Usually, a plurality of cutters and a plurality of tubular frames will be provided with matched pairs of cutters and frames used for forming tubular prostheses having different dimensions. The system may further include a mandrel for holding the tissue as the tissue is attached to the frame, and may still further include a cross-linking agent or other stabilizing agent or preservative for treating the tissue prior to attachment to the frame. The frame may comprise any of the structures described above.
In another aspect of the present invention, a tubular frame for supporting tissue in a tubular geometry with a substantially unrestricted flow lumen therethrough comprises a first tubular frame component having tissue attachment means thereon. The tubular frame typically has a diameter in the range from 1 m to 30 m, preferably from 3 mm to 25 mm, and a length in the range from 1 cm to 30 cm, preferably from 5 cm to 15 cm. The length will be determined at least in part by the length and amount of tissue available from an individual patient. In some cases, frames even longer than 30 cm might find use, but the resulting longer grafts will rarely be needed in some cases, the length of the tubular frame will be adjustable, for example by cutting a desired length of frame or frame components from a relatively long frame stock.
The frame will usually be composed of a resilient metal, and may comprise either a helical element or a plurality of longitudinally spaced-apart ring elements. Attachment means may comprise any one of a second tubular frame component configured to mate with the first tubular frame component, e.g., a pair of nesting helical frame elements, a plurality of fasteners disposed over the first tubular frame component, a sleeve which is received over the exterior of the first tubular frame component, staples for attaching the tissue to the frame component, or the like. Optionally, two or more frames or frame segments may be linked together to create longer grafts, with the frames or frame segments being interlocked and/or overlapped to create a continuous lumen through the resulting assembly.